Japanese Unexamined Patent Application Publication No. 2003-63072 discloses an invention including a duplex printer that prints a mark for alignment on a margin of roll paper, and performs alignment for printing on both sides, on the basis of this mark. According to the descriptions of the patent literature, the duplex printer that performs printing on both sides of a long continuous sheet, typically using two printers respectively, errors in paper feeding precision may be accumulated after continuous long-term printing, and this may cause positional discrepancies between the image printed on the front side and the image to be printed on the back side. In order to solve this problem, the patent literature also describes that as shown in FIG. 4A of the patent literature, according to the invention, the first printer provides marks 93 invisible to naked eyes at regular distances on the front side of printing paper, whereas as shown in FIG. 4B, the second printer reads the mark 93, finding a print start position on the basis of the mark position, and executes printing on the back side of the printing paper, thereby eliminating the discrepancies between the first and the second printers due to accumulation of precision errors in paper feeding. It is also described here that in the second printer, the sensor 100 reads the mark 93, but it is alternatively possible to provide a sensor such as a photoreflector as a paper end detection unit instead of the sensor 100 so as to detect the end of the printing paper.